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Newsgroups: comp.graphics.algorithms
Path: usenet.ee.pdx.edu!cs.uoregon.edu!sgiblab!swrinde!cs.utexas.edu!uunet!pipex!uknet!dmu.ac.uk!se1ap
From: se1ap@dmu.ac.uk (Alan Parker)
Subject: Texture Mapping again!
Message-ID: <1994Feb7.115343.13939@dmu.ac.uk>
Sender: news@dmu.ac.uk (Usenet News Management)
Nntp-Posting-Host: aether.cms.dmu.ac.uk
Organization: De Montfort University, Leicester, UK
Date: Mon, 7 Feb 1994 11:53:43 GMT
Lines: 269
'
' Texture Mapping example.
' Maps 4 sided picture onto a 4 sided polygon.
'
' Written in GFA basic version 2
'
' By Alan Parker (E-Mail: se1ap@dmu.ac.uk)
' On 1/2/94
'
' This texture mapping method was 'borrowed' from Ben of Chaos.
' I've no idea who originally thought of this method.
'
' There's not many detailed comments, I've already done my best when I
' explained the algorithm before and nobody understood it, now it's up to you!
'
' Note: All variables with % after them are Integers, all other are Reals.
' Y co-ords go from top(0) to bottom(199) of the screen.
' 'picture' refers to the original bitmapped picture.
' 'polygon' refers to the polygon drawn on screen.
' The picture to be mapped must be in the top,left (0,0) corner of the
' screen.
' This is not a perspective map, but you would only have to modify the
' scan convertor to generate the picture x,y points depending on the
' z co-ords of the polygon. The texture mapping routine would stay the
' same.
' I've half managed to do a perspective map, but it's still bugged!
' It will be posted when (if?) I manage to get it working...
' *********************************************
' ST specific code.
' Find screen address
'
Screen_base%=Xbios(2) !ST specific call, get screen base address
'
' Set screen address and set to low resolution
Void Xbios(5,L:Screen_base%,L:Screen_base%,W:0)
'
' Load in the picture to be texture mapped (must be in top,left of screen)
Bload "d:\texturem.dem\tmap_pic.pi1",Screen_base%-34
'
' *********************************************
' General Texture Mapping code.
' Machine independent, probably :-)
'
' Variables and arrays.
'
Dim Left_table%(400,2),Right_table%(400,2) ! Scan converter tables for polygon
'
' p.s - replace both the 400 values above with your max screen height in pixels
'
Dim Poly_points(3,1) ! Array for polygon co-ords, 4 pairs(x,y) co-ords
'
P_width%=96 !original picture width in pixels
P_height%=96 !original picture height in pixels
'
Min_y%=32767 !set initial smallest y co-ord of polygon after rotation
Max_y%=0 !set initial largest y co-ord of polygon after rotation
'
' Main program
'
@Get_polygon_points !get initial polygon points from DATA statements into array
@Find_small_large_y !determine the smallest and the largest y-coord in the poly
'
' Send polygon points to the scan converter
X1%=Poly_points(0,0)
Y1%=Poly_points(0,1)
X2%=Poly_points(1,0)
Y2%=Poly_points(1,1)
@Scan_convert(X1%,Y1%,X2%,Y2%,"top") !scan top of picture
X1%=Poly_points(2,0)
Y1%=Poly_points(2,1)
@Scan_convert(X2%,Y2%,X1%,Y1%,"right") !scan right of picture
X2%=Poly_points(3,0)
Y2%=Poly_points(3,1)
@Scan_convert(X1%,Y1%,X2%,Y2%,"bottom") !scan bottom of picture
X1%=Poly_points(0,0)
Y1%=Poly_points(0,1)
@Scan_convert(X2%,Y2%,X1%,Y1%,"left") !scan left of picture
'
' Do the actual texture mapping
@Texture_map
End
'
' *************************************************************
'
' Procedures
'
Procedure Get_polygon_points
Restore Polygon_points ! start of un-rotated polygon co-ords
'
For Count%=0 To 3
Read Poly_points(Count%,0) ! read x xo-ord
Read Poly_points(Count%,1) ! read y co-ord
Next Count%
Return
'
Procedure Find_small_large_y
For Count%=0 To 3
Y_coord%=Poly_points(Count%,1)
'
If Y_coord%<Min_y% ! is this the new lowest y co-ord?
Min_y%=Y_coord% ! Yes...
Endif
'
If Y_coord%>Max_y% ! is this the new highest y co-ord?
Max_y%=Y_coord% ! Yes...
Endif
Next Count%
Return
'
' This is the actual mapping routine.
' It takes the co-ords that have been calcualted by the scan converter
' and 'traces' accross the original picture in between them looking at the
' pixel colour and then plotting a pixel in that colour in the current position
' within the polygon.
'
Procedure Texture_map
'
For Y%=Min_y% To Max_y%
Poly_x1%=Left_table%(Y%,0) !get left polygon x
P_x1=Left_table%(Y%,1) !get left picture x
P_y1=Left_table%(Y%,2) !get left picture y
'
Poly_x2%=Right_table%(Y%,0) !get right polygon x
P_x2=Right_table%(Y%,1) !get right picture x
P_y2=Right_table%(Y%,2) !get right picture y
Line_width%=Poly_x2%-Poly_x1% !what is the width of this polygon line
Inc Line_width% !QUICK fix so it doesn't do divide by zero
'
Px_add=(P_x2-P_x1)/Line_width% !'squash' picture x_dist into polygon x_dist
Py_add=(P_y2-P_y1)/Line_width% !'squash' picture y_dist into polygon x_dist
'
For X%=Poly_x1% To Poly_x2%
Col%=Point(P_x1,P_y1) ! get colour of pixel at current pos in picture
'
Color Col% ! set draw colour to the determined colour
Plot X%,Y% ! plot the pixel in the correct colour
Add P_x1,Px_add ! move x picture co-ord
Add P_y1,Py_add ! move y picture co-ord
Next X%
'
Next Y%
Return
'
' This procedure calculates the x points for the left side of the polygon
' It also calculates the x,y co-ords of the picture for the left side of the
' polygon.
'
Procedure Scan_left_side(X1%,X2%,Y_top%,Line_height%,P_side$)
Inc Line_height% ! No divide by zero
X_add=(X2%-X1%)/Line_height%
'
If P_side$="top"
Px=P_width%
Py=0
Px_add=-P_width%/Line_height%
Py_add=0
Endif
If P_side$="right"
Px=P_width%
Py=P_height%
Px_add=0
Py_add=-P_height%/Line_height%
Endif
If P_side$="bottom"
Px=0
Py=P_height%
Px_add=P_width%/Line_height%
Py_add=0
Endif
If P_side$="left"
Px=0
Py=0
Px_add=0
Py_add=P_height%/Line_height%
Endif
'
X=X1%
For Y%=0 To Line_height%
Left_table%(Y_top%+Y%,0)=X !polygon x
Left_table%(Y_top%+Y%,1)=Px !picture x
Left_table%(Y_top%+Y%,2)=Py !picture y
Add X,X_add !Next polygon x
Add Px,Px_add !Next picture x
Add Py,Py_add !Next picture y
Next Y%
Return
'
' This procedure calculates the x points for the right side of the polygon
' It also calculates the x,y co-ords of the picture for the right side of the
' polygon.
'
Procedure Scan_right_side(X1%,X2%,Y_top%,Line_height%,P_side$)
Inc Line_height% ! No divide by zero
X_add=(X2%-X1%)/Line_height%
'
If P_side$="top"
Px=0
Py=0
Px_add=P_width%/Line_height%
Py_add=0
Endif
If P_side$="right"
Px=P_width%
Py=0
Px_add=0
Py_add=P_height%/Line_height%
Endif
If P_side$="bottom"
Px=P_width%
Py=P_height%
Px_add=-P_width%/Line_height%
Py_add=0
Endif
If P_side$="left"
Px=0
Py=P_height%
Px_add=0
Py_add=-P_height%/Line_height%
Endif
X=X1%
For Y%=0 To Line_height%
Right_table%(Y_top%+Y%,0)=X !polygon x
Right_table%(Y_top%+Y%,1)=Px !picture x
Right_table%(Y_top%+Y%,2)=Py !picture y
Add X,X_add !Next polygon x
Add Px,Px_add !Next picture x
Add Py,Py_add !Next picture y
Next Y%
Return
'
' The procedure takes lines as defined by x1,y1,x2,y2.
' It also takes a string telling it which side of the picture we are mapping
' The strings are top,right,bottom,left.
' This routine decides which 'side' of the polygon the line is on, and then
' calls the apropriate routine.
'
Procedure Scan_convert(X1%,Y1%,X2%,Y2%,P_side$)
If Y2%<Y1%
Swap X1%,X2% ! swap these variable round so we always scan from top to...
Swap Y1%,Y2% ! ... bottom
Line_height%=Y2%-Y1%
@Scan_left_side(X1%,X2%,Y1%,Line_height%,P_side$)
Else
Line_height%=Y2%-Y1%
@Scan_right_side(X1%,X2%,Y1%,Line_height%,P_side$)
Endif
Return
'
' The points for the polygon.
' These points in the form x,y define the shape of a four sided polygon
' rotated 45 degrees. These are 2d points that would normally come from a 3d
' routine after perspective had been applied.
' The points must be defines clockwise....
'
Polygon_points:
Data 100,100
Data 150,150
Data 100,200
Data 50,150
'
' an alternative polygon to try
'
' Polygon_points:
Data 100,100
Data 150,100
Data 200,100
Data 50,100